1. Field of Invention
This invention relates to rigidized, carboxyalkyl substituted heptamethine cyanine dyes and their use as spectral sensitizers in photothermographic imaging elements. These elements find use in photothermographic articles and constructions.
2. Background of the Invention
Silver halide-containing, photothermographic imaging articles (i.e., heatdevelopable photographic articles) processed with heat, and without liquid development, have been known in the art for many years. These articles, also known as "dry silver" compositions or emulsions, generally comprise a support having coated thereon: (1) a photosensitive compound that generates atomic silver when irradiated, (2) a non-photosensitive, reducible silver source, (3) a reducing agent for the non-photosensitive, reducible silver source, and (4) a binder. The photosensitive compound is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that elemental silver (Ag.degree. ) is a catalyst for the reduction of silver ions, and a progenitor of the photosensitive photographic silver halide may be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions, such as by partial metathesis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075), coprecipitation of silver halide and reducible silver source compound (see, for example, U.S. Pat. No. 3,839,049), and other methods that intimately associate the photosensitive photographic silver halide and the non-photosensitive, reducible silver source.
The non-photosensitive, reducible silver source is a compound that contains silver ions. The preferred non-photosensitive reducible silver source comprises silver salts of long chain aliphatic carboxylic acids, typically having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic compounds, such as silver imidazolates, have been proposed, and U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree. ). The imagewise distribution of these clusters is known in the art as a latent image. This latent image generally is not visible by ordinary means and the photosensitive emulsion must be further processed in order to produce a visible image. The visible image is produced by the reduction of silver ions, which are in catalytic proximity to silver halide grains beating the clusters of silver atoms, i.e. the latent image. This produces a black and white image.
As the visible image is produced entirely by elemental silver (Ag.degree. ), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable in order to reduce the cost of raw materials used in the emulsion.
A variety of ingredients may be added to these basic components to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic emulsions, as described in U.S. Pat. Nos. 3,846,136; 3,994,732; and 4,021,249.
One conventional way of attempting to increase the maximum image density of photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming compounds in the emulsion. Color images can be formed by incorporation of leuco dyes into the emulsion. Leuco dyes are the reduced form of a color-bearing dye. Upon imaging, the leuco dye is oxidized, and the color-bearing dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye enhanced silver image can be produced, as shown, for example, in U.S. Pat. Nos. 3,531,286; 4,187,108; 4,426,441; 4,374,921; and 4,460,681.
Multicolor photothermographic imaging elements typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic elements having at least 2 or 3 distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are well known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747; and Research Disclosure, Mar. 1989, item 29963.
Many cyanine and related dyes are well known for their ability to impart spectral sensitivity to a gelatino silver halide element. The wavelength of peak sensitivity is a function of the dye's wavelength of peak light absorbance. While many such dyes provide some spectral sensitization in photothermographic formulations the dye sensitization is often very inefficient and it is not possible to translate the performance of a dye in gelatino silver halide elements to photothermographic elements. The emulsion making procedures and chemical environment of photothermographic elements are very harsh compared to those of gelatino silver halide elements. The presence of large surface areas of fatty acids and fatty acid salts restricts the surface deposition of sensitizing dyes onto silver halide surfaces and may remove sensitizing dye from the surface of the silver halide grains. The large variations in pressure, temperature, pH and solvency encountered in the preparation of photothermographic formulation aggravate the problem. Thus sensitizing dyes which perform well in gelatino silver halide elements are often inefficient in photothermographic formulations. In general, it has been found that merocyanine dyes are superior to cyanine dyes in photothermographic formulations as disclosed, for example, in British Patent No 1,325,312 and U.S. Pat. No. 3,719,495.
Attempts to sensitize at the far red end of spectrum have produced somewhat variable results. In particular, the use of cyanine dyes to impart sensitivity in photothermographic elements in the far red and near infrared has given results quite inconsistent with the performance of such dyes in conventional gelatino silver halide elements. The art therefore leads towards modifying merocyanines. There are however very few merocyanines capable of absorbing at more than 750 nm and also there is uncertainty as to whether dyes which absorb at such wavelengths will also sensitize.
The recent commercial availability of relatively high powered semiconductor light sources, and particularly laser diodes which emit in the red and near-infrared region of the electromagnetic spectrum, as sources for output of electronically stored image data onto photosensitive film or paper is becoming increasingly widespread. This has led to a need for high quality imaging articles which are sensitive in the near infrared region and has created a need to sensitize photothermographic elements to match such exposure sources. In particular, it is necessary to match sources emitting in the wavelength range from 780 to 850 nm, which is towards the extreme end of sensitizing dye art. Such articles find particular utility in laser scanning.
Although spectral sensitizing dyes for photothermographic elements are now known which absorb in the 780-850 nm wavelength range, such dyes are often unstable (i.e., decompose) during storage in the coated film. Thus, a need exists for photothermographic spectral sensitizing dyes which absorb at 780-850 nm and which have improved shelf-life stability. Increased sensitivity and contrast in such dyes would also be desirable.